1. Field of the Invention
The present invention relates to a tunnel magnetoresistance effect element. The ferromagnetic tunnel magnetoresistance effect element is, among magnetoresistance effect films for reading the magnetic field intensity of a magnetic recording medium or the like as a signal, an element which is capable of reading a small magnetic field change as a greater electrical resistance change signal. The ferromagnetic tunnel magnetoresistance effect element is mainly incorporated in, for example, a hard disk drive.
2. Description of the Related Art
Following the high densification of hard disks (HD), highly sensitive magnetic heads with high outputs have been demanded. In response to these demands, attention has been paid to a ferromagnetic tunnel magnetoresistance effect element having a multilayered structure composed of ferromagnetic layer/tunnel barrier layer/ferromagnetic layer, which utilizes a ferromagnetic tunnel magnetoresistance effect.
The ferromagnetic tunnel magnetoresistance effect is a phenomenon that when a current is applied in a laminate direction between a pair of ferromagnetic layers which sandwich a tunnel barrier layer, a tunnel current flowing in the tunnel barrier layer changes depending on a relative angle of magnetization between both ferromagnetic layers.
In this case, the tunnel barrier layer is a thin insulation film which allows electrons to pass therethrough while keeping spins of the electrons due to the tunnel magnetoresistance effect. Generally, the tunnel barrier layer is obtained by oxidizing a thin metal, such as Al, layer of about 10 xc3x85 in thickness.
When the relative angle of magnetization between both ferromagnetic layers which sandwich the tunnel barrier layer therebetween is decreased, the tunneling probability is increased and, therefore, the resistance to current flowing therebetween is decreased. In contrast, when the relative angle of magnetization between both ferromagnetic layers is large, the tunneling probability is lowered, thus, the resistance to current flowing therebetween is increased.
When applying the TMR element to a HDD head, it is essential to lower the electrical resistance of the element. The reason is as follows: Specifically, the resistance of a TMR element is basically expressed by the following equation (1).
R"sgr"=C"sgr"exp(2xcexad)
xcexa=(2mxcfx86/h2)xc2xdxe2x80x83xe2x80x83(1)
wherein d represents a thickness of a barrier layer, xcfx86 represents a magnitude of a barrier potential measured from the Fermi level, and C"sgr" represents an amount determined by an electron state of an insulation layer and magnetic layers, and may be considered to be an amount which is approximately proportional to the product of the Fermi levels of the two magnetic layers.
According to the forgoing equation (1), it is understood that lowering of the resistance of the element can be achieved by reducing the thickness d of the barrier layer. By reducing the resistance of the element, a larger current is allowed to supply, thus, a greater output can be achieved. In addition, in order to eliminate Electro-Static Discharges, it is desirable to lower the resistance of the element.
However, when decreasing the thickness d of the barrier layer, it is expected that the surface condition of the barrier layer should be highly smoother. When the surface is not smooth enough while the thickness of the barrier layer is thin, pinholes are apt to occur in a portion of the barrier layer, and a leakage current may occur through the pinholes. When the leakage current occurs, the greater output can not be achieved. Further, an output may not be obtained when an amount of leakage current is too large.
One approach for smoothing the barrier layer is to smooth a layer under the barrier layer prior to depositing the barrier layer. However, in the prior art abrading method as a smoothing technique, which employs abrasive liquid containing alumina abrasive grain or colloidal silica, smoothness in the order of several angstrom is not expectable, therefore, at this stage, the abrading method does not directly contribute to improvement of the element. Further, also in view of corrosion of an electrode or pinning layer (antiferromagnetic layer), such a polishing method is not preferable.
The present invention has been made under these circumstances and has an object to provide a tunnel magnetoresistance effect element having improved characteristics, particularly having a high TMR for achieving improved head output.
For solving the foregoing problems, according to one aspect of the present invention, there is provided a tunnel magnetoresistance effect element comprising a tunnel multilayered film on an under layer, the tunnel multilayered film having a tunnel barrier layer, a ferromagnetic free layer and a ferromagnetic pinned layer such that the tunnel barrier layer is held between the ferromagnetic free layer and the ferromagnetic pinned layer, wherein three indexes representing a surface roughness state of a surface, which faces the tunnel multilayered film, of the under layer are set such that Ra xe2x89xa60.5 nm, Rmax xe2x89xa65 nm and Rrms xe2x89xa60.55 nm, wherein Ra is one of the three indexes and represents the center line average roughness, Rmax is one of the three indexes and represents the maximum height, and Rrms is one of the three indexes and represents the standard deviation roughness.
It is preferable that the center line average roughness Ra is set to be in the range of 0.001 nm to 0.5 nm, the maximum height Rmax is set to be in the range of 0.01 nm to 5 nm, and the standard deviation roughness Rrms is set to be in the range of 0.001 nm to 0.55 nm.
It is preferable that the surface of the under layer is smoothed by a gas cluster ion beam method.
It is preferable that a gas used in the gas cluster ion beam method is selected from Ar, Ne, Xe, Kr, He, H2, or a mixture thereof.
It is preferable that the smoothing process used in the gas cluster ion beam method is executed under an accelerating voltage of 10-20 keV and 1015-1017 dose.
It is preferable that the under layer is made of W, Ta, Rh, Ti, Cr, Mo, Zr, Hf, or Pt.
It is preferable that the tunnel magnetoresistance effect element comprises the ferromagnetic free layer, the tunnel barrier layer, the ferromagnetic pinned layer and a pinning layer for pinning magnetization of the ferromagnetic pinned layer, which are stacked, in the order named, on the under layer.
It is preferable that the tunnel magnetoresistance effect element comprises a pinning layer for pinning magnetization of the ferromagnetic pinned layer, the ferromagnetic pinned layer, the tunnel barrier layer and the ferromagnetic free layer, which are stacked, in the order named, on the under layer.
It is preferable that a bias magnetic field is applied to the ferromagnetic free layer in a longitudinal direction thereof by biasing means provided at both ends of the ferromagnetic free layer in the longitudinal direction thereof.
It is preferable that the ferromagnetic free layer is a synthetic ferrimagnet.
It is preferable that the ferromagnetic pinned layer is a synthetic ferrimagnet.
It is preferable that the tunnel multilayered film is electrically connected to a pair of electrodes which are oppositely positioned to sandwich the tunnel multilayered film therebetween.
It is preferable that a pair of shield layers are oppositely positioned to sandwich the pair of electrodes therebetween.
According to another aspect of the present invention, there is provided a magnet-resistive tunnel junction head comprising a tunnel magnetoresistance effect element which includes a tunnel multilayered film on an under layer, the tunnel multilayered film having a tunnel barrier layer, a ferromagnetic free layer and a ferromagnetic pinned layer such that the tunnel barrier layer is held between the ferromagnetic free layer and the ferromagnetic pinned layer, wherein three indexes representing a surface roughness state of a surface, which faces the tunnel multilayered film, of the under layer are set such that Ra xe2x89xa60.5 nm, Rmax xe2x89xa65 nm and Rrms xe2x89xa60.55 nm, wherein Ra is one of the three indexes and represents the center line average roughness, Rmax is one of the three indexes and represents the maximum height, and Rrms is one of the three indexes and represents the standard deviation roughness.